After installing "WordPress Landing Pages" plugin, on my site I saw that optin forms I had created with "Custom Contact Forms" plugin were removed (one of them simply disappeared; the other was substituted with an error message).
So I uninstalled "WordPress Landing Pages", and the result was even worst: my home page didn't work anymore (the only elements you could see there was the header image and the message "Fatal error: Call to undefined function wps_custom_form() in /home/[domain.com]/public_html/wp-content/themes/[theme_name]/header.php
I tried to disable the "Custom Contact Forms" plugin, but with no result.
I thought to be able to solve the problem restoring the backuped files of my site, but it didn't work anyway!! Fortunately a friend of mine's friend modified header.php and now it works..
So what I think I can suggest is... be carefull to install "WordPress landing pages plugin" if you have "Costom Contact Forms" installed..